This invention relates to a self-sustained glow discharge gas laser which is preionized by a short-pulse electron beam.
The advent of TEA (transversely excited atmospheric) lasers around 1969 led to the rapid development and better understanding of the self-sustained, high pressure glow discharges that pump these lasers. The key factor in applying a self-sustained discharge to laser pumping is the ability to maintain electric conduction in a volumetrically diffuse or "glow discharge" mode. This is a condition that provides efficient laser pumping for many gaseous laser systems. Laser transitions are available at discrete frequencies spanning the range from the ultraviolet to the far infrared. Notable examples include electronic transitions of the rare-gas halide excimers (e.g., KrF, XeCl, XeF) in the ultraviolet, electronic transitions of the metal halides (e.g., HgBr, HgCl) in the visible, and vibrational transitions of molecules (e.g., HF, CO, CO.sub.2) in the infrared.
In recent TEA laser research, emphasis has shifted to the study of the glow mode of the discharge that is necessary for effective laser pumping. This research has been aimed an increasing the output energy and/or output power of these lasers and increasing their electrical-to-optical energy conversion efficiency.
The requirement for a diffuse glow discharge arises because molecules in the upper laser level are produced by electron collisions. Consequently, the electrons must be much hotter than the gas molecules in order to produce a population inversion. A diffuse glow discharge provides precisely these conditions.
The energy and power of TEA lasers scale with the laser discharge volume and the laser gas pressure. At a given output power level the laser energy can be increased by stretching the laser pulselength. However, attempts to scale up these parameters have been hindered by the tendency for arc formation at high gas pressure. When a filamentary arc forms, the discharge voltage tends to collapse because the arc mode is typically much more conductive than the glow mode. Even if the voltage does not collapse when the arc forms, laser pumping tends to stop because the discharge input energy is channeled into the arc where the electron and gas temperatures rapidly equilibrate. Arcing also destroys the optical quality of the laser medium and can damage system components.
A typical high pressure gas laser consists of a pre-ionization source, two discharge electrodes, and a discharge power supply. Two basic types of preionizers which have been used are ultraviolet radiation where the photon energy is comparable to the ionization potential for the common gases (i.e., 10-25 eV) and sources of high energy radiation where the photon or particle energy is on the order of 0.1-1 MeV. Two common ultraviolet radiation sources are corona discharges and spark discharges. High energy radiation including particles, neutrons, high energy electrons, and x-rays, is used to preionize the gaseous medium of gaseous lasers. If the initial preionized electron density is low, a method of "overvoltage" avalanche ionization is required for self-sustained operation. If a strong preionization source (an electron beam for example) is utilized, the initial preionized electron density can be high enough to eliminate the need for "overvoltage" avalanche ionization.
An attempt has been made to utilize the electron-beam generators as a preionization source. This use was suggested by Levatter and Bradford in 1978 and was used by Bychkov et al in 1979 as an approach to the volume scaling of self-sustained discharge pumped rare-gas halide lasers. However, the electron beams were reported to have the disadvantage, when used as a preionization source, of tending to produce an ionization density that is spatially nonuniform. Nonuniform preionization in the direction transverse to the applied field in a nonuniform self-sustained discharge produces a spatially nonuniform self-sustained discharge that is not well suited for laser pumping (this attempt is reported in Applied Atomic Collision Physics, Volume 3: Gas Lasers, edited by E. W. McDaniel and William L. Nighan, published in 1982 by Academic Press, a subsidiary of Harcourt Brace Jovanovich in New York, pages 396 and 397.)